The purpose of this research is to study the mechanisms of sensory transmission (chemical transmission) from chemoreceptor cells to afferent neurons in the carotid body. Two lines of approach to this study are proposed and pilot experiments have been initiated: (1) Since the existence of a 'sensory synapse' in the carotid body has been recently challenged, we have undertaken a study to determine whether the chemoreceptor fibers of the carotid nerve indeed terminate on the glomus cells of the carotid body, or as free nerve endings scattered throughout the tissue. In this problem, we have applied H3-proline to the sensory ganglion (petrosal) of the carotid nerve and examined the axoplasmic flow of this label along the nerve fibers and into their terminals in the carotid body, using liquid scintillation counting plus light and EM autoradiography. (2) Since preliminary results suggest that a 'sensory synapse' is present in the carotid body, a study of the role of acetylcholine and dopamine as possible transmitters at this synapse is proposed. Pilot experiments have already begun for acetylcholine, to study (a) the uptake of choline and the synthesis and localization of acetylcholine; (b), the effects of natural stimuli on the storage and release of acetylcholine. For this purpose a 'drop-chamber' has been constructed which will allow the recording of chemoreceptor discharges while collecting the acetylcholine released from the carotid body; and (c), the presence and localization of acetylcholine receptors in the carotid body, using reversible and irreversible binding agents, such as alpha-bungarotoxin, dexetimide, nicotine, etc. Similar type experiments are proposed for dopamine. The methods employed in these studies include electrophysiological techniques, neurochemical techniques (electrophoresis, radiometric and gas chromatographic/mass spectrometric determinations), and cyto-labelling techniques (autoradiography and histochemistry).